High frequency testing apparatus



May 5, 1942. J. R. STEEGSTRA HIGH FREQUENCY TESTING APPARATUS Filed April 10. 1940 IiC-Saurce Fig. 2.

Z4 Z6 30 32 3a /i. G Source WITNESSES:

INVENTOR John 7?. Steegszra.

ATTO NEY Patented May 5, 1942 PAT ENT OFFICE HIGH FREQUENCY TESTING APPARATUS JohnzRl. Steegstra; Bridgeport; Gonna, assignor to:

Westinghouse Electric & Manufacturing- Comapany, East. Bit-tsburgh, Pal, a corporation of Pennsylvania- Appiication Apcil 10, 1940,S:=:1'ial No.*328;861

4 Claims,

The present invention relatesto electrical testing, and it has particular relationship-to apparatus for the high frequency testing of I insulated conductors and coils;

In order to satisfactorily testthe insulation'of electrical coils for incipient faults and-weak spots; it has been-ioundnecessary to apply a potential considerably in excess of the normal operating voltage. A high frequencytest circuit for this purpose is disclosed in-United- States Patent No. 1,792,320 to Peters et al.'- In-this patent the circuit including the winding'is-preierably a tuned circuit in order that boththe magnitude and frequency of the testing current will vary in the case of'a faulty windingbyrea' son of leakage of current between the turns of the coil.- The variations from normal may be detected 'by means of a resonant circuit containing an ammeteror-other curreriteresponsive device, the circuit being so related to the winding under test as' to-recei'veenergy by magnetic induction or radiation. or both.

More specifically,'the Peters et a'l..'pa tent discloses a circuit in which a highpotential transformer charges a condenser which, upon'reaching a predetermined potentiah, discharges through a spark gap toprovideanoscillating current. This oscillating current. is. passed through a coil undergoing ,test anditsreaction is observed .to determine. the condition of the. insulation. Despite. extensive. experiments,. it.has been found difiicult to. control this circuit-soas to obtain a spark. which .was always satisfactory. For a cheap portable-set, the costof an accurate rotating. gapis, of course, prohibitive. According to the presentinvention, however, a 0011-: denser is placed in series with the spark gap. This modification causesithe circuit to provide a steady and intense spark which greatly improves its operation.

It is accordingly an object of the invention to provide a novel and improved means for testing electrical insulation.

It is another object of the invention to provide an improved circuit for testing insulation of electrical coils by means of a high frequency test potential.

A further object of the invention is to provide an improvement on the test circuit of the aforesaid Peters et al. patent.

Other objects and advantages of the invention will become evident from the following detailed description taken in conjunction with the accompanying drawing, in which;

Figure 1 is a diagrammatic view of-testing apparatus embodying the invention; and

Fig. 2 is a similar view of another embodiment of the invention.

Referring to Fig. 1, a high potential, high reactance transformer" 2' includes a primarywinding 4 and a secondary winding 6; The primary 4 is energized from an alternating-current source, while the secondary 6- has-a condenser 10 connected 'across it. The transformer" may be ,de-- signed'to'have a secondary potential in "the order of-2000 to 220,000 volts or more whengthe primary 4'is energized bya lowpotential; suchas the usual volts'power circuit: P'aralleling; the condenser H1 is aestationaryspark gap 2' .which is designed to provide a path for the condenser discharge when its potential reaches a predetermined magnitude. A coil I4', thejginsulation of which is to be tested, is connected across the spark gap l2 in series circuit with: azsec'ond condenser I5 and a radio'irequencyammeter18*.1 A

calibrated or voltage measuring sparkigap'; 20is connected in parallel with the" coil 14. The ammeter and the voltage spark gapare provided to indicatethe'condition-of thecoil |'4"in' a manner described hereinafter;

In Fig. 2, a primary winding '24 of a transformer 22 is similarly energized from analternating current source while its secondary 26 has a condenser 30 connected across it. Spark gap 32 is connected in parallel. with the condenser till-to permit energization of a. coil.34"..through condenser 36. Also in series with the coil. 34 undergoing testis an inductance coil designated as 38.

A-wavemeter circuit, designated by 40' is employed to indicate the reactan'ce of the coil 34; This wav'emeter'is similar in construction and operation to that employed in Patent No. 1,792,320. It consists of an adjustably mounted winding 42 disposed adjacent to the coil 38 and connected in series with a current-responsive device 44, a variable condenser 46, and a variable resistance 48.

This wavemeter circuit is adjusted to be in resonance with the high frequency current traversing the winding 34 and coil 38. This adjustment is carried on by first connecting a considerable resistance in at 48 with the coil 42 disposed fairly close to the coil 38. The condenser 46 is then adjusted until the ammeter 44 shows the circuit containing the two windings to be in resonance. The winding 40 is then moved away from the coil 33 and the resistance 48 reduced to increase the sharpness. of the tun g of the coupled circuit. The adjustment is continued in this manner until the winding 421s so related to the winding 38 that the current in the coupled circuit causes full-scale deflection of the instrument 44 with no resistance in the circuit at 48.

It is preferred to tune the wavemeter 40 from the coil 38 of fixed inductance as shown because the test coil 34 may be of unusual shape and size and difficult to position with respect thereto.

However, it may be desirable to tune from the,

test coil itself in some cases, such as when it is used alone in the oscillating circuit or because of other unusual conditions.

Referring to Fig. 2 the op'eration of the circuit may be described as follows: The winding 26 of the transformer 22 charges the condenser 30 to a potential which breaks down the spark gap 32.,

Whenthis occurs, the resistance of this gap is lowered by ionization and an oscillating discharge takes place with a frequency which is a function of the constants of the circuit including the condenser 30, acondenser 36, inductance 38, and the coil undergoing test 34. j

After the wavemeter has been calibrated with a good coil 34 in place, the coils to'be tested are substituted therefor. A short circuit across a large portion of the turns of a coil undergoing test may be detected by the rapid increase in the current resulting therefrom. In the case of a short circuit across only a small portion of the coil, the frequency of the oscillations will change because of the change in inductance of the circuit. Thus the wavemeter 40, being tuned to the original frequency, will be out of resonance and indicate a fault even though the current and voltage change in the oscillating circuit is too small 1 I to detect.

, tion of small short circuits than the wavemeter arrangement of Fig. 2, although it may be preferred for some types of tests.

It has been found that blowing an air stream over the spark gaps indicated at I2 and 32 serves to deionize these gaps between discharges and also to cool them sufficiently to prevent overheating and burning.

Withthe circuit described, it has been found that a steady uniform discharge may be obtained through the respective spark gaps. This steady discharge produces a uniform and constant train of strongly damped waves that are easily detected and read by either of the methods illustrated. The presence of the condenser at [6 or 36 serves to keep the meter reading steady, thereby providing more accurate test results. In addition, the arrangement may be readily embodied in an inexpensive portable test set operating from an ordinary power circuit.

Although the invention has been described with reference to specific embodiments, it will be evi dent that modifications may be made therein. Consequently, it is intended that the invention be limited only by the following claims.

I claim as my invention:

1. In combination, an insulated electrical winding, means for producing an oscillating discharge, said means including a spark gap and condenser arranged to discharge therethrough, a second condenser, means for connecting said winding and said second condenser in series circuit across said discharge producing means for energization thereby, and means for indicating the reaction of said winding to said discharge.

2. In combination, an insulated electrical winding, an alternating current source, .means for, producing a high frequency oscillating discharge, said means including a stationary spark gap and condenser means energized by said alternating current supply to discharge through said gap, conductor means connecting said winding for energization by said oscillating discharge means, a condenser interposed in said conductor means in series circuit with said winding, and means for indicating the reaction of said winding to said oscillating discharge. 5

3. Means for testing the insulation of an elec trical winding comprising an alternating current ond condenser in series circuit across said spark gap to be acted upon by the discharge therethrough, and means for indicating the reaction of said winding to said discharge.

4. Means for testing the insulation of an electrical winding comprising an alternating current source, afirst condenser connected. across said source to be charged thereby, a stationary spark gap connected across said condenser to form a discharge path for said condenser upon its charge reaching a predetermined potential, a second condenser, an inductance coil, means for connecting said Winding, said second condenser and said coil in series circuit across said spark gap to be acted upon by the discharge therethrough, and means for receiving the energy radiated from said coil to indicate the frequency thereof.

' JOHN R. STEEGSTRA. 

